The degree of oxygen saturation of hemoglobin, SpO.sub.2, in arterial blood is often a vital index of the condition of a patient. As blood is pulsed through the lungs by the heart action, a certain percentage of the deoxyhemoglobin, RHb, picks up oxygen so as to become oxyhemoglobin, HbO.sub.2. From the lungs, the blood passes through the arterial system until it reaches the capillaries at which point a portion of the HbO.sub.2 gives up its oxygen to support the life processes in adjacent cells.
By medical definition, the oxygen saturation level is the percentage of HbO.sub.2 divided by the total hemoglobin; therefore, SpO.sub.2 =HbO.sub.2 /(RHb+HbO.sub.2). The saturation value is a very important physiological value. A healthy, conscious person will have an oxygen saturation of approximately 96 to 98%. A person can lose consciousness or suffer permanent brain damage if that person's oxygen saturation value falls to very low levels for extended periods of time. Because of the importance of the oxygen saturation value, "Pulse oximetry has been recommended as a standard of care for every general anesthetic." Kevin K. Tremper & Steven J. Barker, Pulse Oximetry, Anesthesiology, January 1989, at 98.
An oximeter determines the saturation value by analyzing the change in color of the blood. When radiant energy interacts with a liquid, certain wavelengths may be selectively absorbed by particles which are dissolved therein. For a given path length that the light traverses through the liquid, Beer's law (the Beer-Lambert or Bouguer-Beer relation) indicates that the relative reduction in radiation power (P/Po) at a given wavelength is an inverse logarithmic function of the concentration of the solute in the liquid that absorbs that wavelength.
For a solution of oxygenated human hemoglobin, the extinction coefficient maximum is at a wavelength of about 577 nm (green) O. W. Van Assendelft, Spectrophotometry of Haemoglobin Derivatives, Charles C. Thomas, Publisher, 1970, Royal Vangorcum LTD., Publisher, Assen, The Netherlands. Instruments that measure this wavelength are capable of delivering clinically useful information as to oxyhemoglobin levels. In addition, the reflectance pulsation spectrum shows a peak at 577 nm as well. Weijia Cui, Lee L. Ostrander, Bok Y. Lee, "In Vivo Reflectance of Blood and Tissue as a Function of Light Wavelength", IEEE Trans. Biom. Eng. 37:6:1990, 632-639.
In general, methods for noninvasively measuring oxygen saturation in arterial blood utilize the relative difference between the electromagnetic radiation absorption coefficient of deoxyhemoglobin, RHb, and that of oxyhemoglobin, HbO.sub.2. The electromagnetic radiation absorption coefficients of RHb and HbO.sub.2 are characteristically tied to the wavelength of the electromagnetic radiation traveling through them.
In practice of the transmittance pulse oximetry technique, the oxygen saturation of hemoglobin in intravascular blood is determined by (1) alternatively illuminating a volume of intravascular blood with electromagnetic radiation of two or more selected wavelengths, e.g., a red (600-700 nm) wavelength and an infrared (800-940 nm) wavelength, (2) detecting the time-varying electromagnetic radiation intensity transmitted through by the intravascular blood for each of the wavelengths, and (3) calculating oxygen saturation values for the patient's blood by applying the Lambert-Beer's transmittance law to the transmitted electromagnetic radiation intensities at the selected wavelengths.
Whereas apparatus is available for making accurate measurements on a sample of blood in a cuvette, it is not always possible or desirable to withdraw blood from a patient, and it obviously impracticable to do so when continuous monitoring is required, such as while the patient is in surgery. Therefore, much effort has been expanded in devising an instrument for making the measurement by noninvasive means.
A critical limitation in prior art noninvasive pulse oximeters is the few number of acceptable sites where a pulse oximeter probe may be placed. Transmittance probes must be placed in an area of the body thin enough to pass the red/infrared frequencies of light from one side of the body part to the other, e.g., ear lobe, finger nail bed, and toe nail bed. Although red/infrared reflectance oximetry probes are known to those skilled in the art, they do not function well because red and infrared wavelengths transmit through the tissue rather than reflect back to the sensor. Therefore, red/infrared reflectance sensor probes are not typically used for many potentially important clinical applications including: use at central body sites (e.g., thigh, abdomen, and back), enhancing poor signals during hypoperfusion, decreasing motion artifact, etc.